Photoelectric stack height detection device

ABSTRACT

An apparatus for sensing changes in the height of a stack of sheet material positioned on a lift table for seriatim advancement of the sheets from the top of the stack to a processing machine, and for controlling the raising of the lift table to position the stack from a predetermined lowered level to a feeding level in response to activation of a photoelectric detection element when the stack is depleted to the predetermined lowered level. An illuminating means projects a light beam onto a movable reflective sensing member resting on top of the stack and the beam is reflected from the sensing member to the photoelectric element in any one of a plurality of paths as determined by the height of the stack. So long as the beam path is directed within a range so as to be intercepted by the photoelectric element, the photoelectric element remains inactive and the stack is between the feeding and the predetermined lowered level. However, as the stack is depleted to the predetermined lowered level the reflected beam is no longer within the range to be intercepted by the sensitive element thereby activating the photoelectric element which, in turn, actuates drive means for raising the lift table to position the stack at the feeding level.

United States Patent [191 Von Liihmann et al.

[ Sept. 2, 1975 1 PHOTOELECTRIC STACK HEIGHT DETECTION DEVICE [75] Inventors: Wolf-Riidiger Von Liihmann, St.

Georgen; Wilhelm Markgraf, Tennenbronn, both of Germany [73] Assignee: Mathias Bauerle GmbH, St.

Georgen Germany [22] Filed: Apr. 12, 1974 [2]] Appl. No.: 460,584

[30] Foreign Application Priority Data June 30, 1973 Germany 2333366 521 US. (:1 271/154; 250/231 R; 250/561 51 1m. 01. ..B65Hl/18 58 Field Of Search 271/152, 153, 154, 155, 271/156, 215, 263, 262, 31, 130; 214/85 A;

Primary E.raminerRichard A. Schacher Assistant Examiner-Bruce H. Stoner, Jr. Attorney, Agent, or FirmSol L. Goldstein [5 7] ABSTRACT An apparatus for sensing changes in the height of a stack of sheet material positioned on a lift table for seriatim advancement of the sheets from the top of the stack to a processing machine, and for controlling the raising of the lift table to position the stack from a predetermined lowered level to a feeding level in response to activation of a photoelectric detection element when the stack is depleted to the predetermined lowered level. An illuminating means projects a light beam onto a movable reflective sensing member resting on top of the stack and the beam is reflected from the sensing member to the photoelectric element in any one of a plurality of paths as determined by the height of the stack. So long as the beam path is directed within a range so as to be intercepted by the photoelectric element, the photoelectric element remains inactive and the stack is between the feeding and the predetermined lowered level. However, as the stack is depleted to the predetermined lowered level the reflected beam is no longer within the range to be intercepted by the sensitive element thereby activating the photoelectric element which, in turn, actuates drive means for raising the lift table to position the stack at the feeding level.

20 Claims, 7 Drawing Figures PATENTEB sEP 2197s SHEET 1 0f 2 IHOTOELECTRIC STACK HEIGHT DETECTION DEVICE BACKGROUND OF THE INVENTION Scanning the height of a stack of sheets by means of v a beam of light directed obliquely onto the plane of the sheets, and utilizing a photoelectric element for detecting the reflected beam of light for determining the height of the stack, is known in the art. Such devices have also been used for determining the presence of a sheet of paper in a transport line, detecting double sheet thicknesses and the like. However, these devices are not completely reliable in'operation particularly when utilized in association with colored or printed paper. Thus, because of the variations caused by a larger or smaller amount of light absorption by the colored or printed paper, the scanning capabilities of these light reflecting devices are limited. Also, folds, creases and- /or the surface quality'of the paper may also adversely effect the operation of such scanning devices.

In high speed sheet feeding devices for feeding various weights of sheets from the top of the stack and equipped with table raising mechanisms for maintaining the stack at a proper feeding level, in order to perform proper feeding operation it is important that the distance between the top sheet of the stack and the feed element be held, as uniformly and constantly as possible, at the proper feeding level. To achieve this, particularly when feeding thin, light-weight sheets necessitates the use of a lift table which can be raised in relatively minute increments. However, the degree of minute incremental movement of the lift table is dependent primarily on the switching hysteresis of the switching sensitivity of the scanning device. Known mechanical-electrical and pneumatic scanning devices do not provide the desired results of minute incremental movement of the lift table because of their relatively slow switching sensitivity. Also, known photoelectric scanning devices in which the light beam is reflected directly onto the sheet are not completely reliable because of their dependency on the color and the structural surface quality of the sheet.

SUMMARY OF THE INVENTION The present invention provides an improved photoelectric scanning or detection apparatus in which a constant degree of reflection is obtained with different qualities and colors of paper without impairing sheet advancement in the feeding of individual sheets from the stack.

It is an object of the present invention to provide a photoelectric scanning apparatus comprising a movable element or sensing member mounted for rotatable and radial movement in contact engagement with the surface to be scanned. The sensing member is provided with a highly polished, mirror surface which serves as a reflector. Optionally, the sensing member may be associated with a mirror which serves as the reflector. In the preferred embodiment, the sensing member comprises a spherical element in the form of a steel ball having a reflective surface. Although a roller having a reflective surface is functionally equivalent to the steel ball as will be described hereinafter an alternate em bodiment, the use of the ball offers simplified construction and thus lower cost.

Another object of the invention is to provide a photoelectric detection means comprising an illuminating means and a photoelectric element, each providing an optical axis lying in a coincident plane, arranged that the optical axes form therebetween an angle of about The angle is approximately symmetrical on either side of a vertical axis of the sensing member defined by a plane perpendicular to a horizontal plane of a stack of sheet material on a lift table. Accordingly, only slight downward movement of the sensing member in response to depletion of the stack by advancement of the sheets therefrom causes the reflected light beam from the surface of the sensing member to be'directed in a path which is not within a range to be intercepted by the optical axis of the photoelectric element thereby conditioning the photoelectric element to an activated state. Activation of the photoelectric element actuates the drive means for lifting the table a controlled amount such that the stack of sheet material is lifted from the predetermined lowered level to the feeding level.

A further object of the invention is to provide a reflective sensing member supported in a retainer means and movable along a guide axis. The sensing member is mounted within a housing containing the illuminating means and the sensitive element so that the radial movement of the sensing memlber along the guide axis occurs only in a predetermined axial direction maintaining the reflection of the illuminating means in a common plane.

A further object of the invention is to provide a photoelectric detection means in which the response sensitivity thereof may be increased by positioning the guide axis of the movable sensing member in the coincident plane of the optical axes of the illuminating means and the photoelectric element at an angle of between 90 and about with respect to the horizontal plane of the lift table.

Another object of the invention is to provide a scanning apparatus having a rapid response sensitivity by positioning the guide axis of the sensing member and the optical axis of the illuminating means such that they form therebetween an angle of about 90. Adjustment means is provided for diagonally positioning the guide axis of the sensing member with respect to the illuminating means and the sensitive element thereby increasing the angle of deflection of the light beam path while maintaining the same distance of movement of the sensing member in the direction of the axis of symmetry of the optical system. This is accomplished by arranging the sensing member for radial movement along the guide axis as well as for movement in a sidewise direction.

Another object of the invention is to rotatably support the retainer means which supports the sensing member on an axis transverse to the coincident plane of the optical axes of the illuminating means and the photoelectric element. A manually operable device is provided for adjusting the retainer means to selectively settable angular positions between the axis of the retainer means and the horizontal plane of the lift table, or between the axis of the retainer means and the coincident optical axes. In this way the response sensitivity of the scanning apparatus can be varied and adjusted.

for example, to the particular quality and/or color of t the sheets being processed. To some extent,, this arrangement can also be attained by utilizing a, roller as a sensing-reflector by supporting the roller on a lever and pivotally mounting the lever on an axle which can be adjusted about the axis of rotation of the roller. Thus, by adjusting the position of the axle, the axis of pivotal movement of the roller and the direction of movement of the roller are also positionable to various selective positions with respect to the incident light beam, i.e., with respect to the optical axis of the illuminating means.

Other objects, features and advantages of the invention will appear hereinafter as the description proceeds.

IN THE DRAWING FIG. 1 is a perspective view of a lift table for supporting and lifting a stack of sheet material to a feeding level including a photoelectric detection means for actuating the raising of the lift table in response to depletion of the stack to a predetermined lowered level in accordance with the present invention;

FIG. 2 is a section of one embodiment of the photoelectric detection means;

FIG. 3 is a section taken substantially along the line designated AA in FIG. 2;

FIG. 4 is a section of a preferred embodiment of the photoelectric detection means;

FIG. 5 is a section taken substantially along the line designated BB in FIG. 4;

FIG. 6 is a schematic representation, partly in section, illustrating the mode of operation of the photoelectric detection means shown in FIGS. 4 and 5; and

FIG. 7 is another alternate embodiment of a photoelectric detection means.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIG. 1 there is shown a lift table 1 supported for vertical movement on chains 2. The chains are guided in columns 3, 4 and 5 and are driven in unison by a drive motor 6. Actuation of the drive motor 6 drives the chains 2 and thus raises the lift table 1 in minute incremental steps in response to depletion of sheet material as the sheets are advanced from a stack 7 on the lift table 1. Although the sheet advancing means may be of any suitable kind, for purposes of illustration there is shown in FIG. 1 a pneumatic feed device comprising a suction roller 8 for advancing the top sheet from the stack 7 in the direction of arrow 9.

The photoelectric detection or scanning apparatus of the present invention is contained within a housing 10 positioned above the top sheet of the stack 7. The housing 10 is supported on an arm 11 secured to a rail 12 mounted in the columns 3 and 4. The scanning apparatus senses changes in the height of the stack and, when activated as will be described hereinafter, actuates the motor 6 for driving the chains 2 which impart incremental lifting movement to the lift table 1, in the direction of arrow 13, to thereby position the stack 7 from a predetermined lowered level to a feeding level defining a reference position therebetween.

In each of the embodiments illustrated in FIGS. 2 6, the photoelectric detection means comprises an illuminating means 14, a sensitive or photoelectric element l5 and a spherical sensing or movable element 16 provided with a highly reflective surface. The illuminating means 14 is contained within a housing 18 and includes an incandescent lamp 19, a perforated screen 20 and a focusing lens 21. The sensitive element comprises a phototransistor or a photodiode contained within a housing 22. An optical axis 23 of the illuminating means 14 forms with an optical axis 24 of the sensitive element 15 an angle a of about The arrangement of the illuminating means 14 and the sensitive element 15 is such that their optical axes 23 and 24 lie in a coincident plane and are symmetrical on either side of a vertical guide axis 25 of the sensing member 16.

The housings l8 and 22 are secured respectively to straps 26 and 27 within the housing 10. the arrangement is such that the optical axes 23 and 24 of the illuminating means 14 and the sensitive element 15 converge and intersect the vertical guide axis 25 of the sensing member 16 at a point where the surface of the sensing member 16 is also intersected by the guide axis 25.

In the arrangement shown in FIGS. 2 and 3, the sensing member 16 is guided for movement in a sidewise direction along the guide axis 25 and is movable vertically within a bore 28 provided in a bottom plate 29 of the housing 10. The vertical axis of the bore 28 coincides with the vertical guide axis 25 of the sensing member 16. Further, the bore 28 is provided at its lower end with an internal circular band 30 for retaining the sensing member 16 within the bore 28 while permitting the sensing member 16 to lie freely on the top of the sheet material of the stack 7.

The preferred embodiment of the photoelectric detection means as illustrated in FIGS. 4 and 5 differs from that of FIGS. 2 and 3 in that the sensing member 16 is guided in a retainer means 31 rather than in the bore 28 of the bottom plate 29. The retainer 31 is also supported in the housing 10 and is supported for movement about a horizontal axis 32 extending transverse to the coincident planes of the optical axes 23 and 24 of the illuminating means 14 and the sensitive element 15 respectively.

The retainer 31 is supported on rotatable bearings 33 and 34, the bearing 34 extending outwardly beyond a wall of the housing 10 for supporting thereon an adjusting knob 36 provided with a pointer 35. The outerwall of the housing 10 is provided with a scale 37 graduated in degrees for visual observation of a particular angular setting of the retainer 31 on the angular guide axis 38. The positioning of the angular guide axis 38 of the retainer 31 supporting the sensing member 16 and the direction of movement of the sensing member 16 are effected through manual positioning of the knob 36. In the position of the retainer 31 as shown in FIG. 4, the angular guide axis 38 forms an angle 3 of about 45 with the horizontal plane of the stack 7, i.e., the angular guide axis 38 defines a plane perpendicular to the optical axis 23 of the illuminating means 14.

With further reference to FIGS. 4 and 5, in order to maintain a lighttight condition within the housing 10 there is provided a plate 40 having a central opening 39 therein to permit the illuminating means 14 to be projected through the opening 39 onto the reflective sur face of the sensing member 16. The illuminating beam is reflected from the sensing member 16 upwardly in a path through the opening 39.

The alternate embodiment illustrated in FIG. 7 utilizes a roller 17 as a reflective sensing element in place of the sensing member 16 described hereinabove. The roller 17 is also provided with a highly polished surface to reflect the illuminating beam projected by the illuminating means 14 from the roller 17 in a path to the sensitive element 15. The roller 17 is rotatably supported on an axle 42 provided in a pivotal lever 41. The lever 41 is supported for pivotal movement on a wall of the housing such that it can pivot about a horizontal axis extending transverse to the direction of sheet advancement from the stack 7. Except for the pivotal movement of the lever 41, the support for the pivot of the lever 41 may be mounted in a fixed position on the wall of the housing 10.

It is preferable, however, to arrange the pivot of the lever 41 for variable adjustment by meanss of a manually operable knob. The lever 41 may be pivoted about an axis extending parallel with the axis of rotation of the roller 17 such that the direction of pivoting motion of the roller 17 may be adjustably set in a manner substantially analogous to the adjustable setting of the angular guide axis 38 as described and illustrated with ref erence to FIGS. 4 and 5.

In this regard, adjustment of the sensitivity of response or the switching hysteresis may also be obtained by the device shown in the alternate embodiment of F lG. 7 by altering the position of the pivot of the lever 41. As the pivot of the lever 41 is moved closer to the vertical guide axis 25, thereby lowering the roller 17, the roller 17 is also displaced in a sidewise direction analogous to the angular positioning of the retainer 31 as described supra. On the other hand, if the pivot of the lever 41 is arranged at the height of the horizontal axis of the roller 17, the roller 17 is movable only in a vertical direction. Such vertical movement of the roller 17 corresponds to the vertical movement of the sensing member 16 when, for example, the angular guide axis 38 coincides with the line of symmetry of the vertical guide axis 25 as described hereinabovc in the alternate embodiment illustrated in FIG. 2.

The operation of the scanning apparatus of the present invention will now be described with reference to FIG. 6. As shown therein, sheets are advanced from the stack 7 from a feeding level indicated by reference character Ill to a predetermined lowered level indicated by the reference character I12. For purposes of this disclosure, the distance between the reference characters Ill and 112 defines a predetermined range of sheets to be advanced from the stack 7 from the predetermined lowered level I12 to the feeding level Ill, and movement of the sensing member 16 within this range identifies a reference position.

With the stack 7 positioned at the feeding level Iil, the movable sensing member l6 supported by the retainer 3l assumes the position shown in full lines in FIG. 6 wherein the vertical guide axis 25 of the sensing member 16 is perpendicular to the horizontal plane of the stack 7 and coincides with the line of symmetry of the optical axes 23 and 24 of the illuminating means l4 and the sensitive element respectively. The upper periphery of the sensing member 16 intersects this line i of symmetry at the point of intersection of the convergence of the two optical axes 23 and 24. Thus, the illuminating beam projected from the illuminating source 14 striking the surface or periphery of the sensing member I6 is reflected and directed in a path to the optical axis 24 of the sensitive element 15.

The sensitive or photoelectric element 15 detects changes in position of the movable element 16 relative to the reference position and is conditionablc to an inactive state in response to intercepting or interrupting the reflected light beam path from the sensing member 16. As long as the reflected light beam path from the sensing member 16 is interrupted by the sensitive ele- I ment 15, the drive means 6 for the lift table 1 is not actuated and the stack 7 is not lifted to a raised position. This operation continues so long as the level of the stack 7 .is within the predetermined range and the sensing member 16 is within the reference position. However, as the sheet material from the stack 7 is advanced and the stack is depleted to the predetermined lowered level I22, the sensing member 16 is also lowered such that the light beam path reflected from the sensing member 16 is no longer intercepted by the optical axis 24 of the sensitive element. In this condition, the sensitive element 15 is conditioned to an activated state and in response thereto actuates the drive means 6 for raising the table 1 and lifting the stack 7 to the feeding level hl whereat the lifting action will be arrested as a result of movement of the sensing member 16 to a raised position wherein the reflected light beam path from the sensing member 16 is again intercepted by the sensitive element 15. At such time the sensitive element 15 is conditioned to an inactive state and in re sponse thereto deactivates the drive means 6 for lifting the table 1.

Still referring to FIG. 6, as the sheet material is ad vanced and the stack 7 is depleted by an amount indicated by the reference character a, from the feeding level ill to the predetermined lowered level I12, the sensing member l6 is lowered along the angular guide axis 38 from the mid-point position M0 to a mid-point position M2 thus moving a distance indicated by the reference character b which is greater than the distance a. in response to this movement, the surface of the sensing member 16 is now in a position indicated in dot-dash lines and the illumination beam projected from the illuminating means 14- is directed onto the surface of the sensing member 16 at a point S2 resulting in a reflection angle ,82 providing a beam path L2 directed well outside of the range of the optical axis 24 to be intercepted by the sensitive element 15.

It is to be noted that the dimension a in the embodiment of FIG. 6 is shown greatly enlarged for purposes of clarity and understanding. In actual practice the dimension a amounts to between 0.01 and about 0.2 to 0.5 mm., dependent upon the feeding or setting arrangement and the thickness of the sheet material being processed.

With the retainer 31 adjusted to a position such that its angular guide axis 38 is vertical and coincides with the vertical guide axis 25 of the sensing member 16, depletion of the sheet material from the stack 7 by the amount a to the level I12 results in vertical displacement of the sensing member 16 by the amount a from the mid-point position M0 to a midpoint position M1. The position of the sensing member 16 corresponding to the mid-point position M1 is indicated in dotted lines in FIG. 6. ln this position, the illumination beam projected from the illuminating means 14 to the surface of the sensing member 16 strikes the sensing member 16 at a point $1 on the surface of the sensing member 16 and is reflected therefrom in a path directed at an angle Bl. Accordingly, with a predetermined dimension for the depletion of the sheet material from the stack 7 as represented by the reference chaaracter a, although the angle B1 is considerably smaller than the angle [32 the reflected beam path Ll is still well outside of the range of the optical axis 24 to be intercepted by the sensitive element 15. Thus, the sensitive element l5 remains in an inactive state and the drive means 6 for lifting the table 1 is not actuated.

From the foregoing, it will be appreciated that the sensitivity of response can be increased or decreased as desired by selected positioning of the angle 7 of the angular guide axis 38 of the retainer 31 supporting the sensing member 16. Through the facilities of the adjustable guide axis 38 the sensitivity of response and switching hysteresis or the size of the incremental steps of lifting movement of the lift table 1 may be accurately set to the particular characteristics of the sheet material to be advanced from the stack 7. y

In all of the embodiments illustrated and described herein, the reflective movable element in the form of the sphere 16 or the roller 17 resting on the top of the stack 7, provides slight frictional resistance to the top sheet in the stack. Because of the weight of the movable element 16 acting against the top sheet of the stack, the movable element 16 does not tend to vibrate or oscillate such as to cause spurious signals or erroneous switching of the photoelectric element as a result of non-uniform projection and reflection of the light beam path.

In summary, the present invention provides a simple but reliable scanning apparatus for detecting and maintaining the height of the stack of sheet material at a proper feeding level. The sensitive device is highly responsive to extremely slight movement of the movable sensing-reflector element as it is lowered in response to depletion of the stack asthe sheets are advanced from the top thereof. This arrangement provides for feeding sheet material of various weights, colors, surface qualities and the like, and for accurately controlling the raising of the lift table in minute incremental steps corresponding to a predetermined amount of stack depletion to thereby maintain the top of the stack within a predetermined range to provide for consistent and uniform advancement of the sheets from the stack.

What is claimed is:

l. A device for sensing changes in the height of a stack of sheet material lying in a first horizontal plane comprising:

a generally arcuately shaped surface reflective element positioned in contact with the upper sheet in the stack;

photoelectric detection means including an optical path comprising an illuminating source and a photosensor positioned relative to one another defining an illuminating beam projection path and a sighting path for the photosensor, which paths are generally normal to one another thereby establishing a reference position for the stack height where the paths meet for detecting changes in position of the arcuate element relative to the reference position; and

retainer means for movably supporting said arcuately shaped surface element relative to said reference position along a guide axis to complete the optical path.

2. A device as set forthin claim 1 in which the photoelectric detection meansis conditionable to an inactive state in response to intercepting the beam path and conditionablc" to an activated state in response to noninterception of the beam path; and

means responsive to activation of the photoelectric detection means for lifting the stack to the reference position.

3. A device as set forth in claim 2 in which the means for lifting the stack comprises a lift table for supporting the stack and drive means for raising the lift table.

4. A device as set forth in claim 1 in which the reflective element comprises a spherical member.

5. The device as claimed in claim 1 in which said reference position lies in a second plane normal to the first plane, said second plane containing a verticaal reference axis; and mounting means permitting rotational movement of said retainer so that said guide axis can be angularly adjusted relative to the vertical axis.

6. The device as set forth in claim 5 in which the guide axis coincides with the vertical reference axis.

7. The device as set forth in claim 5 in which the angular position of the guide axis and the horizontal first plane of the stack form an angle therebetween in the range of between and 40.

8. An apparatus as set forth in claim 5 in which the arcuate element comprises a spherical element provided with a reflective surface.

9. The device as claimed in claim 5 wherein the rotation of said retainer is about an axis extending normal to said second plane.

10. The device as claimed in claim 5 wherein said reflective element is a cylinder having a reflective surface and the axis of rotation of the cylinder is parallel to said first plane.

1 1. An apparatus for sensing changes in the height of a stack of sheet material comprising:

a generally arcuately shaped reflective surface in contact with the uppermost sheet in the stack for identifying the height of the stack between a feeding level and a predetermined lower level;

photoelectric detection means including an optical path comprising an illuminating source and a photosensor positioned relative to one another defining an illuminating beam projection path and a sighting path for the photosensor, which paths are generally normal to one another, thereby establishing a reference position for the stack height where the paths meet for detecting changes in position of the light reflective surface relative to the reference position and conditionable to an activated state when the light reflective surface is at the predetermined lower level; and

means responsive to the activation of the photoelectric detection means for lifting the stack to the feeding level.

12. An apparatus as set forth in claim 11 in which the means for lifting the stack comprises a lift table for supporting the stack and drive means for raising the lift table.

13. An apparatus as set forth in claim 11 in which the reflective surface comprises a roller provided with a reflective surface.

14. A device for sensing changes in the height of a stack of sheet material positioned on a power driven table for feeding individual sheets lying in a first horizontal plane to a processing machine comprising:

a generally arcuately shaped reflective surface ele ment in contact with the uppermost sheet in the stack;

photoelectric detection means including an optical path comprising an illuminating source and a photosensor positioned relative to one another defining an illuminating beam projection path and a sighting path for the photosensor. which paths are generally normal to one another thereby establishing a reference position for the stack where the paths meet, for detecting changes in position of the arcuatcly shaped surface element relative to the reference position;

said reference position lying in a second plane normal to said first plane; and

means for supporting said reflective element permitting multi-directional movement of the element and the support along a guide axis relative to said first and second planes.

15. The device as set forth in claim 14 in which the reflective element comprises a spherical element.

16. The device as set forth in claim 14 in which the reflective element comprises a roller.

17. The device as set forth in claim 14 in which the guide axis coincides with the second plane.

18. The device set forth in claim 14 in which the guide axis and the first plane form an angle of between and 40.

19. The device as claimed in claim 14 wherein said reflective element is rotationally contained in said support means and adapted to move along a guide axis generally vertical relative to said first plane.

20. The device as claimed in claim 19 wherein said support is rotatably mounted about an axis of rotation that extends in a direction normal to said second plane. l l l 

1. A device for sensing changes in the height of a stack of sheet material lying in a first horizontal plane comprising: a generally arcuately shaped surface reflective element positioned in contact with the upper sheet in the stack; photoelectric detection means including an optical path comprising an illuminating source and a photosensor positioned relative to one another defining an illuminating beam projection path and a sighting path for the photosensor, which paths are generally normal to one another thereby establishing a reference position for the stack height where the paths meet for detecting changes in position of the arcuate element relative to the reference position; and retainer means for movably supporting said arcuately shaped surface element relative to said reference position along a guide axis to complete the optical path.
 2. A device as set forth in claim 1 in which the photoelectric detection means is conditionable to an inactive state in response to intercepting the beam path and conditionable to an activated state in response to non-interception of the beam path; and means responsive to activation of the photoelectric detection means for lifting the stack to the reference position.
 3. A device as set forth in claim 2 in which the means for lifting the stack comprises a lift table for supporting the stack and drive means for raising the lift table.
 4. A device as set forth in claim 1 in which the reflective element comprises a spherical member.
 5. The device as claimed in claim 1 in which said reference position lies in a second plane normal to the first plane, said second plane containing a verticaal reference axis; and mounting means permitting rotational movement of said retainer so that said guide axis can be angularly adjusted relative to the vertical axis.
 6. The device as set forth in claim 5 in which the guide axis coincides with the vertical reference axis.
 7. The device as set forth in claim 5 in which the angular position of the guide axis and the horizontal first plane of the stack form an angle therebetween in the range of between 90* and 40*.
 8. An apparatus as set forth in claim 5 in which the arcuate element comprises a spherical element provided with a reflective surface.
 9. The device as claimed in claim 5 wherein the rotation of said retainer is about an axis extending normal to said second plane.
 10. The device as claimed in claim 5 wherein said reflective element is a cylinder having a reflective surface and the axis of rotation of the cylinder is parallel to said first plane.
 11. An apparatus for sensing changes in the height of a stack of sheet material comprising: a generally arcuately shaped reflective surface in contact with the uppermost sheet in the stack for identifying the height of the stack between a feeding level and a predetermined lower level; photoelectric detection means including an optical path comprising an illuminating source and a photosensor positioned relative to one another defining an illuminating beam projection path and a sighting path for the photosensor, which paths are generally normal to one another, thereby establishing a reference position for the stack height where the paThs meet for detecting changes in position of the light reflective surface relative to the reference position and conditionable to an activated state when the light reflective surface is at the predetermined lower level; and means responsive to the activation of the photoelectric detection means for lifting the stack to the feeding level.
 12. An apparatus as set forth in claim 11 in which the means for lifting the stack comprises a lift table for supporting the stack and drive means for raising the lift table.
 13. An apparatus as set forth in claim 11 in which the reflective surface comprises a roller provided with a reflective surface.
 14. A device for sensing changes in the height of a stack of sheet material positioned on a power driven table for feeding individual sheets lying in a first horizontal plane to a processing machine comprising: a generally arcuately shaped reflective surface element in contact with the uppermost sheet in the stack; photoelectric detection means including an optical path comprising an illuminating source and a photosensor positioned relative to one another defining an illuminating beam projection path and a sighting path for the photosensor, which paths are generally normal to one another thereby establishing a reference position for the stack where the paths meet, for detecting changes in position of the arcuately shaped surface element relative to the reference position; said reference position lying in a second plane normal to said first plane; and means for supporting said reflective element permitting multi-directional movement of the element and the support along a guide axis relative to said first and second planes.
 15. The device as set forth in claim 14 in which the reflective element comprises a spherical element.
 16. The device as set forth in claim 14 in which the reflective element comprises a roller.
 17. The device as set forth in claim 14 in which the guide axis coincides with the second plane.
 18. The device set forth in claim 14 in which the guide axis and the first plane form an angle of between 90* and 40*.
 19. The device as claimed in claim 14 wherein said reflective element is rotationally contained in said support means and adapted to move along a guide axis generally vertical relative to said first plane.
 20. The device as claimed in claim 19 wherein said support is rotatably mounted about an axis of rotation that extends in a direction normal to said second plane. 